Officials in Austin, Texas, are investigating an “outbreak” of fatal drug overdoses that is spreading across the city.

Austin-Travis County Emergency Medical Services (ATCEMS) and the Austin Police Department (APD) said they had received a total of 51 suspected overdose calls as of Tuesday afternoon, with four confirmed deaths and another four possibly related to the surge, according to local sources related to ABC News affiliate KVUE.

According to KVUE, the overdose patients range in age from 20 to 50 and belong to various ethnic groups. Those who died from overdoses have not been identified, but their ages range from their mid-30s to mid-50s.

The surge in calls began around 9 a.m. CT Monday morning and was concentrated in the downtown area, said Angela Carr, EMS department chief at ATCEMS, during a press conference Monday evening. Calls later came in from across the city, including homes, businesses and areas open to the general public.

Carr said a team was immediately dispatched to the area and Narcan rescue kits were distributed. Narcan is administered as a nasal spray, and the drug's active ingredient – naloxone – can quickly restore breathing if someone suffers an opioid overdose.

According to Dr. Heidi Abraham, ATCEMS deputy medical director, told ATCEMS it typically receives two or three overdose calls per day, but Monday's number of calls represented a 1,000% increase in call volume.

“The trend we're seeing in this group of overdoses is that they are unusually fatal,” she said during the press conference. “We haven’t seen overdoses of this amount for several years.”

Abraham said the overdose patients reported consuming several different types of medications, but did not provide further details about the specific medications.

Christa Steadman, ATCEMS public information officer, said officials suspect the “outbreak” was due to “a new batch in the city,” likely from the same source or sources, as symptoms among patients were similar .

Officials said they will continue to provide updates as they become available.

A record number of Americans have died from drug overdoses. In 2022, there were nearly 108,000 drug overdose deaths, about 1% more than the nearly 107,000 overdose deaths in 2021, according to preliminary data from the Centers for Disease Control and Prevention.

Travis County had the fifth-highest number of drug overdose deaths in 2022, with 308, mostly among white residents.

States have previously said that fentanyl, the synthetic opioid that is 50 to 100 times more powerful than morphine, may be causing a rise in overdoses and overdose deaths.

According to TDSHS, there was preliminary data showing Travis County had 188 deaths related to fentanyl poisoning, the second-highest number behind Harris County, where Houston is located.